The present invention relates to the field of photonics and in particular to arrangements for controlling, routing or switching optical signals by wavelength selection.
An optical circulator as illustrated by way of example in FIG. 1 is a known photonic device for the unidirectional transmission of an optical signal or beam from one port to the next sequential port (i.e. from port 1 to port 2, 2 to 3, 3 to 4, etc), without allowing transmission in the reverse directions (i.e. 2 to 1, 3 to 2, 4 to 3, etc). Thus light input at port 1 is output at port 2, light input at port 2 is output at port 3, etc. Here the term "light" is used to include both visible and non-visible radiation e.g. optical signals suitable for the purposes of photonics. To operate on several beams, as is necessary when handling signals from multiple optical fibres, several optical circulators are conventionally required. To perform an all-optical add-drop function that separates channels carried on different wavelengths in an optical signal (i.e. wavelength division multiplexing (WDM) wavelength channels), would also require a multiplicity of optical circulators.
With the spread of WDM wherein a plurality of wavelength channels is carried by a single optical fibre, the need for a compact and cheap means for independently operating on large numbers of wavelength channels is becoming more acute.
A requirement in wavelength multiplexed multi-channel optical networks is to have fully flexible optical add-drop and cross connect functionality at nodes of the network. This means that any wavelength channel carrying data to that node can be dropped into any one of a number of receivers at that node and that any of the node's transmitters may reuse that wavelength to send data on from that node.
To achieve these functions requires switching and demultiplexing of the WDM wavelength channels. Tuneable reflective optical gratings can be used with optical circulators to sort a signal comprising WDM wavelength channels into two sets of wavelength channels. To carry out the add-drop function with 8, 16, 32 or more channels requires the sorting function to be repeated many times which requires the use of large numbers of filters and optical circulators.